The greatest source of ionizing radiation exposure to the general public today is from naturally occurring airborne radionuclides inside residences. The majority of the natural radioactive material found in the indoor environment is due to the primordial radionuclide, uranium-238, and its associated decay series members. The series member of primary concern is radon-222, a gaseous radioactive pollutant that can accumulate in the air within dwellings, particularly those with low outside air infiltration rates. The immediate precursor of radon-222 is radium-226 which, because of its 1600 year half-life, serves as a virtually continuous source of radon-222 production within residences. All substances of natural origin, such as water, rock, soil, and construction materials which incorporate crustal materials as aggregate, contain some amount of radium-226. The radioactive decay of radium-226 produces the inert radioactive gas, radon, which is then free to diffuse through porous soils and construction materials and eventually to enter the environment within a building. Once inside a dwelling, radon will eventually decay (half life=3.8 days) to initiate what is commonly referred to as the short-lived radon decay product series. This series consists of heavy metal atoms of polonium, lead, and bismuth, each of which is, in turn, radioactive and possesses very short half-lives (i.e., 30 minutes or less). The principal radionuclides in the decay chain are as follows: Rn-222 (radon) decays to Po-218 (RaA) which decays to Pb-214 (RaB) which decays to Bi-214 (RaC), which decays to Po-214(RaC'), which decays to long-lived Pb-210 (RaD).
The airborne radioactive decay products in this series frequently collide with and attach themselves to large dust particles within the air inside a dwelling. The effective size of such attached decay products ranges from 0.05 to a few micrometers in diameter. Depending on the concentration of dust in the room air, up to 20% of the decay products will not attach to dust particles and will remain in a free ion state. Since unattached decay products are often positively charged, they tend to attract other small polar molecules in the air, such as water vapor and trace gases, and are believed to exist, at least temporarily, as very small and highly diffusive ion clusters.
The health hazard associated with the radon decay series stems from the inhalation by humans of both the attached and unattached decay products, and their eventual decay and irradiation of susceptible lung cell populations. Epidemiological studies on uranium miners have demonstrated a causal relationship between the inhalation of high concentrations of radon decay products and an increased risk of lung cancer. The radiobiological consequence of the attachment state of the decay product being inhaled stems from the knowledge that highly diffusive unattached decay product atoms preferentially deposit in the upper segments of the tracheo-bronchial tree of the lungs. This is the site within the lungs where most of the cancers among the uranium miners have occurred. As a result, the unattached decay products are believed to have the potentiality of causing a higher localized lung dose equivalent (and therefore have a higher associated risk) per unit amount of radioactive material inhaled. In contrast, the large attached decay products are deposited rather uniformly throughout the respiratory system, and are considered by radiobiologists to impose a much lesser risk of health damage (cancer) on the exposed individual.
Attempts to remedy the problem of radon decay product exposure fall into two categories: (1) control techniques applied to mine atmospheres and (2) theoretical and limited experimental studies applied to radon and radon decay products in buildings. Control strategies also fall into two basic categories: (1) the prevention of radon gas buildup inside a structure (mine or building), either by providing a barrier to prevent the radon from entering or by selecting foundation and construction materials that are relatively low in their naturally occurring radionuclide content, and (2) the removal of radon decay products from the interior air. As stated above, airborne decay products can exist as either free species or attached to aerosol particles, and different control strategies have been devised for each case.
The traditional method for the control of radon decay products in uranium mines has been mechanical ventilation, and maximum permissible concentrations for such products have been expressed in terms of Working Level (WL).sup.(1). Significant decreases in radon decay product concentrations have been accomplished by recirculating large volumes of air within a mine shaft, without introducing makeup air. Decreases in working levels by factors of 10 to 20 using recirculation rates of 20 and 60 per hour have been reported. The large recirculation rates apparently caused radon decay product removal by both increased deposition on mine surfaces of decay products attached to aerosol particles and increased diffusion of unattached decay products to mine surfaces. FNT .sup.(1) The Working Level (WL) is defined as any combination of short-lived radon decay products, i.e., RaA, RaB, and RaC, per liter of air that upon complete decay will release 1.3.times.10.sup.5 MeV of alpha energy. The most common unit for expressing the time-integrated exposure to the short-lived decay products is the Working Level Month (WLM). FNT Exposure to an atmospheric concentration of one WL for 170 hours (one working month) is defined as a cumulative exposure of 1 WLM.
Both filtration and electrostatic precipitation have also been used for radon decay product removal in mines. [Rock, R. L.; "Control of Radon Daughters in U.S. Underground Uranium Mines", Proceedings of the 12th AEC Air Cleaning Conference, Report CONF-720823, Vol. 1, U.S. Atomic Energy Commission, Washington, DC (Jan. 1973); Goodwin, A.; "Review of Problems and Techniques for Removal of Radon and Radon Daughter Products from Mine Atmospheres", Proceedings of the 12th AEC Air Cleaning Conference, Report CONF-720823, Vol. 1, U.S. Atomic Energy Commission, Washington, DC (Jan. 1973); Washington, R. A.; Chi, W.; and Regan, R.; "The Use of Vermiculite to Control Dust and Radon Daughters in Underground Uranium Mine Air", Proceedings of the 12th AEC Air Cleaning Conference, Report CONF-720823, Vol. 1, U.S. Atomic Energy Commission, Washington, DC (Jan. 1973); Shreve, J. D. and Cleveland, J. E.; "Effects of Depressing Attachment Ratio of Radon Daughters in Uranium Mine Atmosphere", Am. Ind. Hyg. Assoc. J., Vol. 33, No. 4, p. 304 (1972).] For example, Washington et al., (referenced above) reported that use of a deep bed of vermiculite particles as a filter reduced radon decay product levels by 20%-40%, depending on bed configuration and filtration velocity.
Shreve and Cleveland (also referenced above) used high efficiency particulate air (HEPA) filters in an attempt to improve upon the results previously attained with medium-to-low efficiency filters. They measured decreases of 40%-70% in RaA concentrations and 10%-70% in RaC concentrations at various distances downstream from the filter.
Most of the air ceaning devices developed for mines are not directly applicable to the control of radon decay products in residences because of large differences in scale.
Fitzgerald et al. [Fitzgerald, J. E., Jr.; Guimond, R. J.; and Shaw, R. A.; "A Preliminary Evaluation of the Control of Indoor Radon Daughter Levels in New Structures", U.S. Environmental Protection Agency, EPA-520/4-76-018, Washington, DC (Nov. 1976)] conducted an evaluation of measures suitable for controlling radon exhalation through foundations of buildings. They compared the cost effectiveness of four alternative control technologies: polymeric sealants, excavation, crawl space construction, and ventilation. Measures to remove radon decay products once they have penetrated into a structure were grouped together by Fitzgerald et al., and considered under the category "effective ventilation." They defined "effective ventilation" as the replacement of air within a structure with air containing outdoor radon concentrations by natural infiltration or by recycling of air within a structure through an air cleaner. These authors considered the following types of "effective ventilation": (1) increased natural ventilation (2) filtration, (3) electrostatic precipitation, and (4) combined electrostatic precipitation and outside air exchange. Because they could find no data on the ability of air cleaners to reduce radon decay product concentrations, they used typical air cleaner characteristics to model their performance.
The models assumed that natural ventilation was equivalent to 1.0 air change per hour (1 h.sup.-1). Since this level of natural ventilation has a large effect on the concentrations of radon and its decay products, the addition of "effective ventilation" rates of 1 to 2 h.sup.-1 was found to have relatively little additional effect. When the energy usage and periodic maintenance requirements of effective ventilation practices were combined with their relatively poor incremental decay product removal efficiency, their cost effectiveness was found to be poorer than the other alternatives studied.
A report by Windham et al. [Windham, S. T.; Savage, E. D.; and Phillips, C. R.; "The Effects of Home Ventilation Systems on Indoor Radon-Radon Daughter Levels", U.S. Environmental Protection Agency, EPA-520/5-77-011, Washington, DC (Oct. 1978)], summarizes the results of an experimental program to measure the effects of ventilation on radon and radon decay products in an unoccupied single family house located on reclaimed phosphate land in Polk County, Florida. Measurements were made of the effects on radon and its decay product concentrations of using a central air conditioner, central blower without air conditioning, and outside air ventilation.
These experiments indicated that all three control techniques significantly decreased radon and radon decay products levels below those measured when the house was sealed and allowed to reach a steady state. Each technique caused the WL to decrease by a factor of about ten.
The sealed house had a natural ventilation rate (infiltration) of 0.5-0.6 air changes per hour (h.sup.-1) running the air conditioner or central fan increased this to 2.0-2.5 h.sup.-1, whereas using a window fan to introduce outside ventilation resulted in a ventilation rate of 5.4 h.sup.-1. The authors concluded that the decrease in WL measured for all three techniques was probably caused in each case by the increased ventilation rate. They did not believe that factors such as filtration by the air conditioner filter or plate-out on the various surfaces in the house contributed significantly to the WL reduction, but no data were collected to confirm this opinion.
Holub et al. [Holub, R. F.; Droullard, R. F.; Ho, W.; Hopke, P. K.; Parsley, R.; and Stukel, J. J.; "The Reduction of Airborne Radon Daughter Concentration by Plateout on an Air Mixing Fan", Health Physics, Vol. 36, No. 4, p. 497 (1979)] introduced radon into a test chamber and measured the effects of an air mixing fan on radon decay product concentrations. They found that the operation of the fan decreased radon decay product levels in the air by a factor of about two. The initial hypothesis was that the air motion would cause the radon decay products to plate-out on the walls of the chamber, causing the observed decrease in air concentrations. They found, however, that the decrease in concentration was due entirely to deposition on the fan blades themselves rather than on the walls.
The use of turbulent convection (air mixing fans) has been reported by Rudnick et al. (Rudnick, S. N.; Hinds, W. C.; Maher, E. F.; Price, J. M.; Fugimoto, K.; Gu, F. and First, M. W.; "Effects of Indoor Air Circulation Systems on Radon Decay Product Concentrations", Final Report on USEPA Contract Number 68-01-6050, February 1982, U.S. Environmental Protection Agency, Washington, D.C.) as effective in removing airborne unattached decay products which typically have diameters in the range of 0.001 to 0.01 micrometers. The highly diffusive nature of particles in this size range favors their removal from the air space by deposition onto surfaces by molecular diffusion. The turbulent flow created by the fan facilitates such deposition. Air turbulence reduces the boundary layer thickness at the surface to air interface throughout a room and thus reduces the distance that unattached decay products must travel by molecular diffusion before depositing onto room surfaces. The net result is a higher flux of unattached decay products plating onto the walls of a room and a corresponding reduction in the airborne concentrations of such decay products. Enhanced surface deposition caused by turbulent convection becomes progressively less effective as particle size increases and is relatively unimportant for particle sizes greater than 0.1 micrometers.
As may be seen by the above, past efforts at particle control to reduce radon working levels suffer from two major defects: (1) most of the experimental work has been performed in uranium mines rather than in buildings (or test chambers that simulate buildings), and (2) studies of the effectiveness of control devices in buildings were not performed in a reproducible manner, so that optimal design and operation of air cleaning devices could be adequately devised.